(1) Field of the Invention
The present invention relates to an ink-jet image forming apparatus for forming images on recording paper by ejection of ink droplets. In particular, the present invention relates to a method of performing stable image formation by keeping the distance between the recording head and recording paper constant at all times without regard to the type of recording paper.
(2) Description of the Prior Art
In general, in an image forming apparatus of an ink-jet type(to be referred to hereinbelow as ink-jet printers) which can form micro dots at high densities while making relatively low noise during printing, image forming is performed by ejecting ink droplets onto the surface of recording sheets which are fed successively. More specifically, a generally known scenario is that the recording head, which is set with an ink cartridge on a carriage, ejects ink droplets onto the recording paper that moves intermittently in the auxiliary direction while the carriage reciprocates in the main scan direction that is perpendicular to the direction of conveyance of the recording paper.
As the ink cartridge, one which supplies black ink and color inks, i.e., yellow, cyan and magenta is provided. Accordingly, an ink-jet printer not only produces text printing with black ink but also is able to perform full color printing by adjusting the ratio of the amounts of color inks ejected toward the recording paper.
In ink-jet printers of this type, the distance between the position of the ink nozzle of the ink head and the recording paper surface (head to medium gap) can be considered one of the critical factors that determine the print quality.
Since the sheets to be used for ink-jet printers are generally diverse, from typical recording paper (e.g., about 0.05 to 0.2 mm thick) to postcards, envelopes, etc., the distance from the ink nozzle position of the recording head to the recording paper surface varies due to variations in sheet thickness. Further, an ink droplet ejected from the ink nozzle to the recording paper varies in its shape and speed depending on the head to medium gap. For these reasons, setting of the head to medium gap is critically important to obtain the shape and speed of ink droplets suitable for printing.
Illustratively, when the head to medium gap is narrow, the speed at which an ink droplet reaches the recording paper remains high, so that ink droplet splashes on the recording paper surface. That is, ink may scatter over the non-printing area beyond the area on the sheet that the ink droplet should be ejected onto for printing, or may contaminate the recording head.
On the other hand, when the head to medium gap is wide, the actual travel route of the ink droplet ejected from the carriage whilst it is being moved deviates from the expected path. Since the point at which the ink droplet arrives thus deviates from the proper position, this will cause distortion in the resulting image.
In order to prevent degradation of print quality due to inappropriate head to medium gap, there have been many proposals up to now for adjusting the head to medium gap in the technical field of ink-jet printers.
For example, there is a method of adjustment as to the head to medium gap by manually switching the height of the recording head in accordance with the thickness of the recording paper to be used for printing. This method involves selection modes such as xe2x80x98thick paper modexe2x80x99, xe2x80x98normal paper modexe2x80x99, xe2x80x98thin paper modexe2x80x99, etc., so as to allow the user to select the desired mode with a lever handle.
Japanese Patent Application Laid-Open Hei 9 No.109460 discloses an ink-jet printer configuration in which the carriage or recording head is moved in accordance with the head to medium gap detected by a sensor so as to adjust the distance to the user-desired text size to be printed.
However, as to the configuration in which the height of the recording head is manually selected, the user needs to decide the thickness of the recording paper in order to obtain beneficial print quality. Accordingly, this configuration needs complicated print handling and potentially can cause breakdown of the recording head due to user""s misjudgment. When switching of the height of the recording head is performed by shifting the carriage up and down, the whole carriage configuration inevitably becomes complicated, which leads to a marked rise in manufacturing cost. Further, the change of the height of the carriage could hinder the stability of the scanning movement of the carriage in the main scan direction and may cause degradation of image quality. Further, in the waiting mode the carriage is positioned at a maintenance station located at one side with respect to the main scan direction, however there is a possibility that the carriage cannot be set at an appropriate position relative the maintenance station when the height is changed. In order to avoid this, it is necessary to make the height of the maintenance station adjustable or provide other measures, hence the result is complexity of the entire printer.
In the configuration disclosed in the above publication, part of the sensor for detecting the head to medium gap and the mechanical arrangement for adjusting the head to medium gap are mounted on the carriage for moving the recording head. Therefore, the carriage needs to carry the sensor and the mechanical arrangement together with the recording head, thus consuming extra electric energy.
The present invention has been devised in view of the above problems, it is therefore an object of the present invention to provide an ink-jet image forming apparatus with which the distance between the recording head and recording paper can be kept constant at any time regardless of the type of recording paper without increasing in any way the complexity of the carriage configuration and without inducing any increase in power consumption.
In order to achieve the above object, in the present invention, the platen for guiding the recording paper is moved in the direction away from the recording head so as to set the gap between the recording head and recording paper at a proper distance.
Specifically, the invention is assumed to involve an ink-jet type image forming apparatus having a platen for guiding recording media and a recording head disposed opposing the platen for ejecting ink droplets onto recording media. This ink-jet type image forming apparatus is provided with a platen actuating means which moves the platen so as to adjust the gap between the recording head and the recording medium.
By this limitation, it becomes possible to adjust the distance between the ink nozzle position of the recording head and the recording medium surface on the platen (the head to medium gap) and set it constant by moving the platen which will serve as a guide table for recording media during printing(during image forming) and keep the recording medium flat and adjust the height of the recording medium relative to the ink nozzle. With this arrangement, it is possible to perform printing of different types of recording media having different thicknesses under the same conditions. In the present invention, since the head to medium gap can be set correctly without shifting the recording head or the carriage having the recording head mounted thereon, there is no need to provide a motor or other parts in order to drive the recording head or the carriage up and down. Therefore, it is possible to avoid increase of the carriage in weight, hence increase in power consumption can be inhibited while the carriage can be moved smoothly. Here, examples of applicable recording media include, recording paper, OHP film and any other material as long as it is printable by sprayed ink.
Specific examples of the platen actuating means include the following configurations. First, the ink-jet type image forming apparatus may further include paired conveyer rollers for holding the recording medium being image formed by ejected ink droplets from the recording head and conveying it. In this arrangement, the platen actuating means is adapted to move the platen in linkage with the holding operation of the recording medium between the paired conveyer rollers and set the gap between the recording head and the recording medium at a predetermined distance.
Also, the ink-jet type image forming apparatus may further include a thickness detecting means for detecting the thickness of the recording medium from the clearance between the paired conveyer rollers when a recording medium has been held between the pairs of rollers. In this arrangement, the platen actuating means is adapted to move the platen based on the thickness of the recording medium detected by the thickness detecting means and set the gap between the recording head and the recording medium at a predetermined distance.
Each of the platen actuating means is configured to move the platen so as to keep the gap between the recording head and the recording medium always constant regardless of the thickness of the recording medium.
The platen actuating means specifically operates in the following manners. First, the platen actuating means, in linkage with the movement of the paired conveyer rollers separating from one another by the thickness of the recording medium when a recording medium is held between the paired conveyers rollers, moves the platen by the distance substantially corresponding to the movement of separation to thereby set the gap between the recording head and the recording medium at the predetermined distance.
As another action, the platen actuating means, using the force arising when the paired conveyer rollers separate from one another by the thickness of the recording medium when a recording medium is held between the paired conveyers rollers, drives the platen by the distance substantially corresponding to the movement of separation to thereby set the gap between the recording head and the recording medium at the predetermined distance.
These limitations enable the platen actuating means to operate so as to keep the gap between the recording head and the recording medium surface constant regardless of the type (the thickness) of the recording medium, hence it is possible to keep the travel time of an ink droplet from its being ejected from the recording head to its arrival at the recording medium constant. Therefore it is possible to place the ink droplet at the designated position, leading to improvement of the quality of image forming. It is also possible to avoid the recording medium, during conveyance, interfering with the recording head and hence being mis-fed or damaging the recording head to too short a gap between the recording head and recording medium.
The paired conveyer rollers are arranged at a position upstream of the recording head with respect to the direction of conveyance of the recording medium. By this limitation, the recording medium has already been held between the paired conveyer rollers when the recording medium reaches the position where it opposes the recording head. That is, image forming by ink droplets being ejected from the recording head to the recording medium is carried out when the platen movement in linkage with the holding between these paired conveyer rollers has been completed. As a result, it is possible to perform beneficial image forming from the leading part (the downstream end with respect to the direction of conveyance) of the recording medium.
Further specific examples of the platen actuating means include the following configurations. That is, the platen actuating means may include a linkage mechanism for linking the platen actuating movement with the movement of the paired conveyer rollers separating one from the other. This linkage mechanism may include: a first linkage for axially supporting a first conveyer roller of the paired conveyer rollers; and a second linkage which is coupled with the first linkage and presses and moves the platen in linkage with the movement of the first linkage as the first conveyer roller moves when a recording medium is held by the paired conveyer rollers.
Thus, these limitations enables a relatively simple configuration, that is, the linkage mechanism, to realize a mechanism for always keeping the gap between the recording head and the recording medium surface constant.
The means for supporting the platen actuating means can be configured as follows. First, a platen clutching means for keeping the platen at a position at which the platen has been shifted by the platen actuating means may be provided.
By this limitation, the position of the platen having been shifted can be retained by the platen clutching means even after the holding of the recording medium between the paired conveyer rollers is released. Therefore, it is possible to keep the proper gap from the recording medium to the recording head even at the rear part of the recording medium (the upstream part with respect to the conveying direction), hence perform beneficial image forming.
Specific examples of the platen clutching means include the following configurations. The platen clutching means may be comprised of paired discharge rollers arranged on the output side of the platen and may be configured such that one of the discharge rollers is rotationally supported by the platen. Further, the paired discharge rollers may be adapted to hold the recording medium and convey it when the holding of the recording medium by the paired conveyer rollers is released.
By these limitations, when the recording medium is being held between by the discharge rollers, one of the discharge rollers moves together with the platen by the distance corresponding to the thickness of the recording medium. This movement of the platen makes it possible to keep the proper gap between the recording medium and the recording head.
Other specific examples of the platen clutching means are configured as follows. That is, the platen clutching means may have a movement direction limiting element which allows the platen to move only in the direction away from the recording head.
The platen clutching means may have a releasing means for releasing the constraint imposed by the movement direction limiting element so as to allow the platen to move closer and back to the recording head. This releasing means is comprised of a release roller put in sliding contact with the movement direction limiting element and a drive roller rotating the release roller in linkage with the paper feed roller so that the constraint imposed by the movement direction limiting element is released by the release roller sliding into contact with the movement direction limiting element as the drive roller turns in linkage with the movement of the paper feed roller. Further, the movement direction limiting element may be comprised of a one-way clutch mechanism.
By these limitations, the position of the platen having been shifted can be retained even after the holding of the recording medium between the paired conveyer rollers is released. Further, provision of the releasing means always makes it possible to move the platen by the distance corresponding to the thickness of an individual recording medium.
When each of the above configurations further includes a movement limiting means for limiting the amount of movement of the platen by the platen actuating means, it is possible to avoid platen deformation which would be caused when the platen moved further than needed.
The platen actuating means may have a linkage mechanism for coupling the movement of the platen in linkage with the movement of the paired conveyer rollers when they separate from one another while the movement limiting means is adapted to abut this linkage mechanism to limit the movement of the linkage mechanism. This arrangement enables a relatively simple configuration to avoid the platen moving further than needed.
In the case where a bowing regulating structure for covering the non-image forming area on the recording medium is provided so that the gap between the recording head and the surface of the recording medium conveyed over the platen will be prevented from becoming narrower than a predetermined distance, it is possible to keep the head to medium gap equal to or wider than the predetermined distance. This provides an assisting function for setting the head to medium gap properly. Further, since the bowing regulating structure is arranged so as to cover the non-image forming area on the recording medium, the presence of this bowing regulating structure will never interfere with the printing operation.
The paired conveyer rollers comprise a drive roller and follower roller, the linkage mechanism has an adjuster element which is pivotally supported by the machine housing with its one end rotatably supporting the follower roller and the other end coupled with the platen. When a recording medium is held between the paired conveyer rollers, the follower roller moves by the thickness of the medium relative to the drive roller, whereby the adjuster element pivots following the movement of the follower roller to cause the platen to move away from the recording head.
In this configuration, when the adjuster element is configured so that the distance of movement of the platen with the rotation of the adjuster element is adapted to be equal to the thickness of the recording medium, the follower roller moves by the distance corresponding to the thickness of the recording medium when the recording medium is held between the paired conveyer rollers. This movement causes the adjuster element to turn so that the platen moves away from the recording head. The amount of movement of the platen is greater as the recording medium is thicker, thus it is possible to move the platen properly in accordance with the thickness.
In the case where an elastic element which provides thrust to cause the platen to approach the recording head is arranged, even if the amount of movement of the follower roller becomes greater than the thickness of the recording medium (even if the platen would move down excessively), it is possible to adjust the platen at a proper position by the thrust from the elastic element.
In the case where a feed direction regulating structure on which the recording paper side parallel with the direction of conveyance of the recording paper being conveyed over the platen is abutted, is provided, so as to prevent the recording paper from skewing or moving zigzagedly, the recording medium is prevented from moving sidewards or obliquely. As a result, it is possible to perform image forming always under fixed conditions whilst the recording medium being stably conveyed.
Another specific example of the platen actuating means may include a platen support structure for supporting the platen by placement thereon, so that the platen will move down by gravity as the platen support structure moves down away from the recording head, to thereby set the gap between the recording head and the recording medium at a predetermined distance.
This configuration also makes it possible to perform printing of different types of recording media having different thicknesses under the same conditions. Further, in the present invention, since fixed print conditions can be established without shifting the recording head or the carriage having the recording head mounted thereon, there is no need to provide a motor or other parts in order to drive the recording head or the carriage up and down. Therefore, it is possible to avoid increase of the carriage in weight, hence increase in power consumption can be inhibited while the carriage can be moved smoothly.
This configuration may further includes: paired conveyer rollers for holding therebetween a recording medium being image formed by ejected ink droplets from the recording head and conveying it; a thickness detecting means for detecting the thickness of the recording medium from the clearance between the conveyer rollers when a recording medium has been held between the pairs of rollers, wherein the platen actuating means moves the platen support structure downward based on the thickness of the recording medium detected by the thickness detecting means and sets the gap between the recording head and the recording medium at a predetermined distance.
Further, the platen support structure may be adapted to support the underside of the platen at multiple sites. This configuration is able to prevent the platen from being twisted and prevent the upper surface of the platen from being positioned at different heights when the platen is moved up and down. Therefore, the head to medium gap can be set to be practically uniform across the entire printing area, thus making it possible to perform printing over the whole page of the recording medium under the same conditions.
Also, the paired conveyer rollers may comprise a drive roller and follower roller while an adjuster element which is pivotally supported by the machine housing with its one end rotatably supporting the follower roller and the other end coupled with the platen support structure is provided. When a recording medium is held between the paired conveyer rollers, the follower roller moves by the thickness of the medium relative to the drive roller. The adjuster element pivots following the movement of the follower roller so that the platen support structure moves downward to thereby set the gap between the recording head and the recording medium at the predetermined distance.
In this case, the adjuster element is configured so as to cause the platen support structure to move downwards one to one and half times greater than the distance of movement of the follower roller relative to the drive roller.
In this case, when a recording medium has become held between the paired conveyer rollers, the thickness of recording medium decreases slightly from the pressure by the paired conveyer rollers. In order to compensate this, the distance of downward movement of the platen support element is set to be 1.0 to 1.5 times of the distance of movement of the follower roller, so that the amount of pivot of the adjuster element will be suitable for the thickness of the recording medium.
Since an elastic element which provides upward thrust to cause the platen support structure to approach the recording head is provided, even if the amount of movement of the follower roller becomes greater than the thickness of the recording medium (even if the platen would move down excessively), it is possible to adjust the platen at a proper position by the thrust from the elastic element.
In the case where the thrust from the elastic element is adapted to press the follower roller of the paired conveyer rollers against the drive roller, a relatively thick recording medium will be held between the conveyer rollers with a greater grip, whereas a relatively thin recording medium will be held between the conveyer rollers with a lower grip. In this way, it is possible to convey a recording medium by a proper holding force in accordance with the thickness of the recording medium.
In the case where a platen position limiting structure for limiting the position of the platen so as to keep the gap between the platen upper surface and recording head equal to or shorter than the predetermined distance is provided, even if the amount of movement of the platen is too large(even if the platen would move upward excessively), it is possible to limit the platen position limiting structure within the predetermined range, so as to prevent the platen from interfering with the ink nozzles of the recording head.
The position, on the platen, limited by the platen position limiting structure is arranged directly above the platen support site by the platen support structure. Therefore, by arranging the constraint point and support point at almost the same area, it is possible to prevent the platen from being twisted and prevent the upper surface of the platen from being positioned at different heights, hence prevent the platen from interfering with the ink nozzles of the recording head.
Finally, the position, on the platen, limited by the platen position limiting structure is arranged at a position away from the feed area of the recording media on the platen. Thus it is possible to limit the position of the platen without damaging the recording medium being conveyed over the platen.